Organisation of the amygdalo-thalamic pathways in rats

Abstract 

This study examines the organisation of the pathways from the amygdala to the thalamus. Amygdaloid nuclei (medial, central, basolateral and olfactory groups) of Sprague-Dawley rats were injected with biotinylated dextran using stereotaxic coordinates and their brains were then aldehyde-fixed and processed using standard methods. We have three major findings. First, the amygdala has a distinct set of projections to particular nuclei of the thalamus. The thalamic nuclei with the heaviest amygdaloid terminations include the zona incerta, the mediodorsal and the midline nuclei. Second, nuclei of different amygdaloid groups project to the thalamus in slightly different patterns. For example, some groups of nuclei project to the thalamic reticular nucleus (e.g. medial, olfactory) whilst others do not (e.g. central, basolateral). Thus, there is a certain amount of heterogeneity within the amygdaloid projections to the thalamus. Third, when we compare our results on the amygdalo-thalamic pathways to the many previous descriptions of the thalamo-amygdaloid pathways, we note that they are largely out of register. In other words, some of the thalamic nuclei that project to a given group of amygdaloid nuclei do not necessarily receive a projection back from that same amygdaloid nucleus. Hence, there is no substrate for a strong feed-back relationship between the thalamus and the amygdala, as there has been shown for other centres of the brain (e.g. between the thalamus and neocortex).